Fiber optic communications networks, optical data buses and optical beamformers call for joining of single optical fibers to multiple fibers. In particular, frequency division multiplexing (FDM) requires efficient optical splitting and combining. Typical couplers are selected from direction couplers, star couplers and power splitters.
Directional couplers are based on parallel cores of single-mode fibers, where the cladding is partially removed, or parallel single-mode waveguides on a substrate. Optical tunneling provides the means for energy transfer where parts of the core or waveguides are brought close enough together to obtain the desired coupling fraction. Directional couplers can be used for simple distribution of power, for example, from one fiber equally into two fibers.
Branching devices can be made on a substrate where the waveguide splits into two fibers to branch off small portions of energy, or power splitting can be achieved with the aid of filters. Such devices are subject to losses, and the aperture angle should be kept as small as possible, on the order of 1.degree. . When such a device is used to combine optical signals with the same wavelength a loss of 3-dB occurs.
Star couplers can be made for splitting and combining more than two optical signals using a cascade of either branching devices or directional couplers. However, the branching devices have high loss due to the 3-dB loss each time two signals are combined. Directional couplings are difficult to manufacture, at least monolithically, due to the existence of crossings.
It would be desirable to provide a device and method for combining a number of optical signals carried by separate fibers which has a low insertion loss and is relatively easy to manufacture. It is also desirable to provide a complimentary device and method for splitting off a number of optical signals from a single fiber with low loss. It is to such a device and method that the present invention is directed.